1. Field of the Invention
The invention relates to an abnormality determination apparatus that performs abnormality determination for a determination object device based on a detection signal of an oxygen sensor provided in an exhaust passage of a multi-cylinder internal combustion engine.
2. Description of Related Art
Japanese Patent Application Publication No. 2014-95321 discloses that an internal combustion engine is provided with a fuel injection valve (passage injection valve) that is a type to inject fuel into an intake passage, and a fuel injection valve (cylinder injection valve) that is a type to directly inject the fuel into a cylinder. Such an internal combustion engine feeds the fuel into the cylinder, by concurrently executing the drive control of the passage injection valve and the drive control of the cylinder injection valve, depending on the operation state.
Further, JP 2014-95321A discloses an internal combustion engine that can use gasoline and alcohol as the fuel. The internal combustion engine can perform an operation of using only alcohol or only gasoline as the fuel, and in addition, can perform an operation of using a mixed fuel in which alcohol and gasoline are mixed at an arbitrary proportion.
In recent years, there has been practically used an abnormality determination apparatus that performs abnormality determination for a determination object device (a fuel injection valve, an exhaust gas purification catalyst, or the like) based on a detection signal of an oxygen sensor provided in an exhaust passage of a multi-cylinder internal combustion engine. As such an abnormality determination apparatus, for example, there is known an apparatus that determines the occurrence of an abnormality (imbalance abnormality) in which the air-fuel ratio of air-fuel mixture becomes unequal (imbalanced) among the cylinders of the internal combustion engine. The imbalance of the air-fuel ratio of the air-fuel mixture among the cylinders of the internal combustion engine increases the fluctuation in the air-fuel ratio of the air-fuel mixture in one combustion cycle of the internal combustion engine. For example, the above abnormality determination apparatus forms a temporal waveform of the air-fuel ratio of the air-fuel mixture, based on the detection signal of the oxygen sensor, and determines the occurrence of the imbalance abnormality when the change rate of the air-fuel ratio in the temporal waveform is high.